Add mocking support

[aronisstav]

When applying Concuerror on large software projects it is desirable to be able to use mocks, in order to not modify application code.

Features:

• have arbitrary code in mocks
• mock non-exported functions (not important)

Ideas:

• Use eproxus/meck: rejected, as meck mocks use communication with one gen_server per mocked module, which has further calls to cover (so it cannot easily be instrumented) and introduces ordering/races.
• Use a new option:
  • Name: mock
  • Arguments:
    • the name of an exported, 0-arity function returning a list of
      • {From :: mfa(), To :: fun()} tuples